Roman emperor Gratian issues a new law exempting leaders of Christian churches from "compulsory public service," which may have meant or at least included most taxation.
The Roman government didn't just impose monetary taxes on people, but also required uncompensated public service for the state and community (like filling a public office), sometimes causing the person to incur great personal costs.
The new Roman law states:
"We order that priests, deacons, sub-deacons, exorcists, altar attendants, and acolytes shall be exempt from personal employments." [CTh 16.2.24]